1. Field of the Invention
The present invention relates to an endoscope having a flexible tube. More particularly, the present invention relates to an endoscope having a flexible tube with a coil spring, and in which entry of the flexible tube into a body cavity can be safely performed without influence of undulating motion due to the coil spring.
2. Description Related to the Prior Art
An endoscope includes a grip handle and an elongated tube disposed to extend from the grip handle in a distal direction. The elongated tube is entered in a body cavity of a body, for example, gastrointestinal tract, and includes a rigid tip device, a steering device and a flexible device of a great length. The tip device includes a viewing window and lighting windows disposed at a distal end surface. The steering device is moved by rotation of steering wheels at the grip handle, and bends in upward and downward directions and right hand and left-hand directions. A flexible tube structure is included in the flexible device.
In the flexible device, a direction, orientation and the like of the tip device is difficult to set due to the flexibility. Advance of the tip device to an object of interest in the body cavity is difficult. In view of this, a known type of the endoscope includes a stiffness controller, which is disposed in the flexible tube structure for adjusting flexibility relative to a direction of bending the flexible device. The stiffness controller includes a coil spring and a control wire. The coil spring has a large length and includes coil turns contacting one another. The control wire is disposed through the coil spring. A distal end of the control wire is retained to the coil spring. The control wire retains a proximal end of the coil spring in a condition of allowing pull of the control wire. A rotatable control wheel is disposed on the grip handle, and rotated to pull the control wire, so that stiffness of the flexible device is adjusted by changing a compressed condition of the coil spring.
U.S. Pat. No. 5,885,208 (corresponding to JP-A 10-305005) discloses an example of the stiffness controller for the endoscope. An amount of pull of the control wire is settable for preventing buckling of the coil spring even upon bending the flexible tube structure at 180 degrees with a smallest curvature on a condition of a maximum level of bending stiffness by applying compression force to the coil spring by pulling the control wire. Also, JP-A 2003-275167 discloses another example of the endoscope in which a joint device is disposed between a variable stiffness portion and a portion of a constant stiffness and does not deform plastically even with a smaller bend than the variable stiffness portion and the portion of the constant stiffness. This is for the purpose of raising durability at the time of concentration of bend at the portion with a large difference between the levels of the stiffness.
In the course of entry of the elongated tube into a body cavity, the flexible device of the endoscope having the stiffness controller is set in a somewhat flexible condition in a first portion of a travel path. According to an increase in a length of the entry in the travel path, the stiffness is increased to set the flexible device in a nearly straight form for the purpose of effectively transmitting force of steering, so as to raise the manipulability for entry.
The coil spring for use in the stiffness controller has a sufficiently large length, and includes numerous coil turns of a strand which adjacently contact one another. In case the coil spring is compressed, fine unevenness occurs between the coil turns of the strand even while the flexible device is not flexed. Undulating motion of the coil spring is likely to occur with changes with convexity and concavity in an axial direction.
Upon occurrence of the undulating motion, a convex portion of the coil spring is pushed to the outside more than a concave portion of the coil spring. Reaction force to the compression force is created and results in the bending stiffness of a higher level than the inner side. There occurs variability in the bending stiffness in a circumferential direction, namely variability of the stiffness in radial directions even with an equal point in relation to the axial direction. This variability in the stiffness in the flexible device according to a direction of the bending is likely to give unexpected grip feeling to an operator manipulating the endoscope.